The present invention relates in general to reproduction apparatus including an intermediate image transfer member wherein a marking particle image is transferred from a primary image forming member to the intermediate image transfer member and then to a receiver member, and more particularly to an endless web mechanism for facilitating transfer of a marking particle image from the intermediate transfer member to the receiver member which may be a paper or plastic sheet upon which the image is to be fixed.
In modern high speed/high quality electrostatographic reproduction apparatus (copier/duplicators or printers), a latent image charge pattern is formed on a uniformly charged dielectric support member. Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the support. The dielectric support is then brought into contact with a receiver member and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support and the image is fixed to the receiver member by heat and/or pressure to form a permanent reproduction thereon.
Application of the electric field to effect marking particle transfer is generally accomplished by ion emission from a corona charger onto the receiver member while in contact with the dielectric support, or by an electrically biased roller urging the receiver member against the dielectric support. Roller transfer apparatus offer certain advantages over corona transfer apparatus in that the roller transfer apparatus substantially eliminate defects in the transferred image due to paper cockle or marking particle flakes. This result stems from the fact that the pressure of the roller urging the receiver member against the dielectric support is remarkably efficient in providing intimate uniform contact therebetween. Moreover, in color systems, a receiver sheet can be attached to a roller and the roller rotated to bring the sheet through transfer relationship with a primary image member. An electric field between the drum and the image member superposes a series of single color images on the sheet creating a multicolor image. See, for example, U.S. Pat. No. 4,712,906, Bothner et al, issued Dec. 15, 1987 which is representative of a large number of references in commercial apparatus using this approach.
U.S. Pat. No. 3,781,105 granted to Meagher Dec. 25, 1973 suggests a backing roller for transferring single color images to a receiver sheet. In this instance the reference suggests that the backing roller have an outside layer or layers of a low intermediate conductivity and that a constant current source be used for establishing an electric field. The intermediate conductivity is established by using material having a resistivity of 10.sup.9 to 10.sup.11 ohm-cm. This material is conductive enough to permit the establishment of an electric field but provides a relatively high impedance which causes the field to be less variable in response to variations in the receiver sheet. With such more resistant materials, receiver sheets can vary between paper and transparency stock and also as to thickness and ambient relative humidity without an unacceptable variation in the field that would cause insufficient transfer in some instances or electrical breakdown in others.
Backing rollers having a resistivity in the neighborhood of 10.sup.10 ohm-cm are commonly made by doping a high resistance polyurethane material with tiny conductive particles such as carbon, iron or other antistatic materials sufficiently to provide the conductivity needed. Although such backing rollers having a high resistivity are considered preferred in such systems, they do generate problems. If the field is provided between two members that roll in contact with each other, the field is constantly being established through that rolling contact. The substantial resistance of the backing roller increases the time constant in establishing the field thereby either increasing the necessary size of the nip for transfer or reducing the speed of the system.
A number of references show the use of intermediates in both single color image formation and multicolor image formation. For example, FIG. 8 of the above mentioned U.S. Pat. No. 4,712,906 shows a series of single color images being formed on a primary image member. The single color images are transferred in registration to an intermediate roller to create a multicolor image on the surface of the roller. A multicolor image is then transferred in a single step to a receiver sheet at a position remote from the primary image member. This system is particularly advantageous in forming multicolor marking particles images, because the receiver sheet does not have to be attached to a roller for recirculation but can be fed along a substantially straight path. It can also be used with single color marking particles image formation for a number of other reasons including facilitating duplex and preventing contact between a primary image member and a receiver sheet which may contaminate the image member with paper fibers and the like.
U.S. Pat. No. 4,931,839 granted to Tompkins et al on Jun. 5, 1990 shows use of an intermediate web of relatively high intermediate conductivity which superposes single color marking particles images by transfer from a primary image member. The images are transferred to a receiver sheet which is backed by a conductive roller. Substantial impedance does not appear to be provided at this transfer to allow for variations in receiver sheet impedance.
In U.S. Pat. No. 5,187,526 granted to Zaretsky on Feb. 16, 1993, there is shown a transfer arrangement with the advantages that are obtained from use of an intermediate, while still handling a variety of receiver sheets and operating at reasonable speed. In this arrangement, an electrostatic image is formed on a primary image member. Marking particles are applied to the electrostatic image to create a marking particles image corresponding to the electrostatic image. The marking particles image is carried by the primary image member into transfer relation with an intermediate image member having a resistivity less than 10.sup.9 ohm-cm while applying an electric field between the image members sufficient to transfer the marking particles image to the intermediate image member. The marking particles image is then brought into transfer relation with a receiver sheet while the receiver sheet is backed by a transfer backing member having a resistivity of 10.sup.10 ohm-cm or greater in the presence of an electric field between the intermediate image member and the transfer backing member urging transfer of the marking particles image to the receiver sheet. The relatively high conductivity of the intermediate image member facilitates efficient transfer of marking particles images from the primary image member to the intermediate image member using a fairly narrow nip. A high resistance intermediate image member is not necessary at this transfer because no receiver sheet is present. At the second transfer in which the receiver sheet is present, impedance is provided by the transfer backing member rather than the intermediate image member and the nip is somewhat longer allowing for the slower rise time of the electric field.
This arrangement is particularly usable in color processes in which the color image is created on the intermediate image member by superposition of a series of single color images formed on the primary image member. Superposition of the single color marking particles images on the intermediate image member is facilitated by a more conductive intermediate image member. The second transfer to the receiver sheet is facilitated by the less conductive transfer backing member in that transfer.
Difficulties in using an intermediate image member are related to controlling the transfer field in the nip area between the intermediate member and the transfer backing member and in achieving reliable detack of a receiver member from the intermediate image member. Marking particle image transfer has heretofore been compromised to ensure transfer field control and detack because marking particle transfer and detack are accomplished with the same roller. The coupling of marking particle transfer and detack is complicated and imparts significant constraints on the design of the intermediate image member, increases the overall cost of the transfer system, and degrades image quality. Moreover, further problems with the intermediate image member are encountered when receiver members become exposed to a wide range of relative humidities, and also when many different receiver member types and weights are used (especially receiver members with low stiffness such as light weight papers).